Fax For Asterisk provides two components: res_fax and res_fax_digium. Res_fax is an Asterisk resource module that adds fax termination and origination functionality in Asterisk. It provides Asterisk dialplan functions and dialplan applications to enable the user to build highly-customizable fax solutions. Res_fax_digium provides core fax processing functionality in the form of several supported fax modems - V.21, V.27ter, V.29, and V.17 - to achieve speeds up to 14400bps.

Fax For Asterisk provides the functionality to send and receive faxes to / from TDM and IP channels - TDM channels are established across Digium telephony boards and IP channels can use regular G.711 audio encoding or T.38 encapsulation. Faxes transmitted and received by Fax For Asterisk begin and end as TIFF image files. TIFF files may be readily converted into or from other formats using standard Linux command-line utilities. Digium's customers of Fax For Asterisk may purchase license keys coded for a specific number of channels.

Each channel of capability allows Fax For Asterisk to initiate one modem session and process one fax session. As customers need additional channel capacity, they may purchase additional channels of fax capacity and register those keys to their existing Asterisk system to use the aggregate number of channels available across all keys registered to a single Asterisk.

One channel of Fax For Asterisk is provided to all customers of Switchvox SMB 4.0 as well as all customers upgrading from SMB 3.5 who maintain active user subscriptions and software maintenance. In order to enable Fax capability in Switchvox, please locate your Switchvox activation code and visit the Switchvox website for instructions on receiving your complimentary Fax For Asterisk license key to unlock Fax capability in Switchvox. Additionally, each open source or commercial Asterisk system is eligible to receive from Digium, a single channel of Fax For Asterisk, for no cost. Free Fax For Asterisk is available to all Asterisk users as a free, zero cost purchase from the Digium webstore.

Only one channel of Free Fax For Asterisk may be used with an installation of Asterisk. If you require multiple channels of Fax capability, you may purchase channels of Fax For Asterisk.